add polyfill for Object.assign as it is not supported by IE11 (#2710)

src/polyfill.js

@@ -1612,4 +1612,36 @@ if (!String.prototype.padEnd) {
     };
 }
 
+// Object.assign polyfill for IE11
+// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Polyfill
+if (typeof Object.assign !== 'function') {
+    // Must be writable: true, enumerable: false, configurable: true
+    Object.defineProperty(Object, "assign", {
+        value: function assign(target, varArgs) { // .length of function is 2
+            'use strict';
+            if (target === null || target === undefined) {
+                throw new TypeError('Cannot convert undefined or null to object');
+            }
+
+            var to = Object(target);
+
+            for (var index = 1; index < arguments.length; index++) {
+                var nextSource = arguments[index];
+
+                if (nextSource !== null && nextSource !== undefined) {
+                    for (var nextKey in nextSource) {
+                        // Avoid bugs when hasOwnProperty is shadowed
+                        if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
+                            to[nextKey] = nextSource[nextKey];
+                        }
+                    }
+                }
+            }
+            return to;
+        },
+        writable: true,
+        configurable: true
+    });
+}
+
 /* jshint ignore:end */